1. Field of the Invention
The present invention relates to connecting method and structure of superconducting wires used for a coil of a superconducting magnet, etc.
2. Description of the Prior Art
Each of the following methods is widely known as a general connecting method of superconducting wires.
(1) A method for overlapping and connecting superconducting wires to each other by soldering or welding.
(2) A method for removing stabilizers from superconducting wires to expose superconducting filaments and overlapping or twisting the superconducting filaments. Otherwise, A method for soldering, welding or melting-injecting an exposed portion of the superconducting filaments.
(3) A method for pressing superconducting filaments by using a sleeve instead of the soldering, welding or melting-injecting method in the above item (2).
The first method (1) is a simplest method. However, since superconducting filaments come in indirect contact with each other, the electric resistance of a connecting portion of the superconducting filaments is large so that this method is not suitable so much for a coil of a superconducting magnet, etc. requiring a persistent current mode.
In the second method (2), the superconducting filaments are directly connected to each other so that the electric resistance of a connecting portion of the superconducting filaments is greatly reduced. Therefore, this second method is used in many conventional examples. However, rigidity of the connecting portion is low so that it is necessary to further improve the second method to obtain a higher critical current value.
In the third method (3), the superconducting filaments are pressed by using the sleeve so that the superconducting filaments come in closer contact with each other and a higher critical current value can be obtained. Further, it is possible to expect higher rigidity of an entire connecting portion of the superconducting filaments. Therefore, this third method is generally used as a recent connecting method of superconducting wires.
FIG. 6 is a perspective view for explaining the construction of a superconducting wire. In FIG. 6, reference numerals 1 and 2 respectively designate a superconducting wire and a superconducting filament. For example, the superconducting filament is constructed by a wire material made of NbTi and having a diameter about 20 to 50 .mu.m. A stabilizer 3 is used to electrically and thermally stabilize the superconducting filament 2 by burying the above superconducting filament 2 into the stabilizer 3. A copper material is often used as the stabilizer 3.
FIG. 7 is a cross-sectional view showing the above conventional connecting method (3) as a method for pressing the superconducting filaments by using the sleeve and shown in e.g., Japanese Laid-Open Patent No. 59-16207. In FIG. 7, reference numerals 1a and 1b respectively designate one superconducting wire and another superconducting wire to be connected. Reference numerals 2a and 2b respectively designate one superconducting filament and another superconducting filament for the superconducting wire 1b. Reference numerals 3a and 3b respectively designate stabilizers for the superconducting wires 1a and 1b. Reference numerals 4a and 4b respectively designate peeling faces of the superconducting wires 1a and 1b from which the above stabilizers 3a and 3b are removed by a corrosive solvent such as nitric acid. For example, a cylindrical sleeve 5 is made of copper and the above superconducting filaments 2a and 2b suitably bundled are inserted into this cylindrical sleeve 5. The above sleeve 5 is pressed in a direction of pressing force shown by an arrow 6. Reference numeral 7 designates a clearance of the superconducting filaments caused in the vicinity of the peeling faces 4a and 4b since no superconducting filaments 2a and 2b come in close contact with each other when the superconducting filaments 2a and 2b are pressed.
FIG. 8 is a cross-sectional view showing a conventional connecting method in which a shape of the sleeve 5 shown in FIG. 7 is partially changed.
In FIG. 8, constructional portions 1a to 4a, 1b to 4b and 5 to 7 are similar to those shown in FIG. 7. Therefore, an explanation about these constructional portions is omitted in the following description. A counter bore 5a is disposed in an inner diameter portion of the sleeve 5 at one end thereof and overlaps the stabilizers 3a and 3b at a length l.sub.1. An arrow 6a designates a direction of pressing force applied to the sleeve 5 in a pressing range of the above length l.sub.1.
FIG. 9 is a cross-sectional view showing another conventional connecting method shown in e.g., Japanese Laid-Open Patent No. 1-260776.
In FIG. 9, constructional portions 1a to 4a, 1b to 4b and 5 to 7 are similar to those shown in FIG. 7. Therefore, an explanation about these constructional portions is omitted in the following description. Taper portions 8a and 8b are formed and inclined at a predetermined angle with respect to peeling faces 4a and 4b from which stabilizers 3a and 3b are removed.
Procedures of the connecting methods will next be explained.
With respect to FIG. 7, the following procedures are carried out.
(1) The stabilizers 3a and 3b are respectively removed from the superconducting wires 1a and 1b to expose the superconducting filaments 2a and 2b.
(2) The exposed superconducting filaments 2a and 2b are suitably bundled.
(3) These superconducting filaments 2a and 2b are respectively inserted into the sleeve 5 until ends of the superconducting filaments 2a and 2b are in conformity with the peeling faces 4a and 4b of the above stabilizers 3a and 3b.
(4) Next, the sleeve 5 is pressed by using a tool such as an unillustrated die in the direction of the arrow 6 with a predetermined pressing force such as several ten tons.
In FIG. 8, the above procedures (1) to (4) with respect to FIG. 7 are similarly carried out. Namely, the superconducting filaments 2a and 2b are respectively pushed and inserted into the sleeve 5 until a deep position of the counter bore 5a such that positions of the peeling faces 4a and 4b of the stabilizers 3a and 3b are in conformity with this deep position.
(5) Thereafter, the sleeve 5 is pressed in the pressing range of the length l.sub.1 by using a tool such as an unillustrated die in the direction of the arrow 6a with a predetermined pressing force. Thus, the sleeve 5 is fixed to the superconducting wires 1a and 1b.
In FIG. 9, the sleeve and the superconducting wires can be connected to each other in procedures approximately similar to the above procedures (1) to (5) with respect to FIG. 8. In this case, the pressed sleeve 5 is molded along the taper portions 8a and 8b on the peeling faces 4a and 4b from which the stabilizers 3a and 3b are removed.
FIGS. 10a and 10b are cross-sectional views showing a state of a peeling face 4 from which a stabilizer 3 is removed by a corrosive solvent such as nitric acid. FIG. 10a corresponds to FIGS. 7 and 8 and FIG. 10b corresponds to FIG. 9. In general, the peeling face 4 removing the stabilizer 3 therefrom is chemically processed as mentioned above. Accordingly, it is difficult to obtain a planar peeling face including a straight line as shown in each of FIGS. 7 to 9. Normally, an irregular peeling face 4 is obtained as shown in FIGS. 10a and 10b.
Accordingly, the conventional connecting method of the superconducting wires 1 each having such an irregular peeling face 4 has the following problems.
Firstly, when the superconducting filaments are pressed by using the sleeve 5, it is easy to cause a portion in which it is difficult to make the superconducting filaments close to each other by irregularities in the vicinity of the irregular peeling face 4. Therefore, many clearances 7 are formed and superconducting characteristics are reduced by these clearances 7.
Namely, the superconducting filaments 2 are slightly vibrated by influences of an applied magnetic field and a vibration thereof in a superconducting state so that the clearances 7 cause generation of heat of the superconducting filaments 2. As a result, a great disadvantage of transfer (called quench) from the superconducting state to a normal conducting state is caused.
Secondly, a portion of the superconducting filaments 2 in the vicinity of the peeling face 4 tends to be disconnected and excessively distorted by deformation of the sleeve 5 at a pressing time. Accordingly, a critical current in a connecting portion is reduced in comparison with a critical current (i.e., an electric current which can flow through the superconducting filaments in the superconducting state) in a range of the superconducting filaments covered with the stabilizer 3, thereby causing a change in superconducting characteristics. Further, no critical current is stabilized so that excessive dispersion in the critical current is caused.
Thirdly, when the superconducting filaments 2 are inserted into the sleeve 5 in a pressing operation, it is not easy to position the sleeve 5 by irregularities of the irregular peeling face 4. Accordingly, no base portions of the peeling face 4 and the sleeve 5 are firmly fixed after the pressing operation, thereby forming an unstable connecting portion. Therefore, it is impossible to provide a suitable rigidity for the connecting portion so that the superconducting filaments 2 tend to be disconnected and distorted. Further, similar to the above-mentioned case, the critical current is reduced and dispersion in the critical current is caused.
To solve these problems and obtain a preferable rigidity of the connecting portion, there is a proposed method in which the sleeve 5 overlaps the stabilizer 3 as shown in FIGS. 8 and 9 to form two separate pressing positions and press the sleeve in these two positions. However, no problems about disconnection and distortion of the superconducting filaments 2 in the vicinity of the peeling face 4 are sufficiently solved.
It is necessary to arrange a connection portion of a coil of a superconducting magnet, etc. in a position having a low magnetic flux density in consideration of the above reduction and dispersion in critical current. Such an arrangement limits design and manufacture of a connecting structure.
Further, when the coil is designed, an electric current density is reduced in consideration of the reduction and the dispersion in critical current so that cost of the coil as a product is increased.